In one type of known micromachined sensing device as shown in U.S. Pat. No. 5,345,824, which is hereby expressly incorporated by reference for all purposes, a mass is suspended over a substrate and is tethered to move along a sensitive axis. The mass has a central beam that is elongated in the direction of the sensitive axis and has fingers extending away from the central beam in a direction perpendicular to the sensitive axis. Each of these fingers is between a pair of stationary fingers, and together each movable finger and its adjacent stationary fingers form a differential capacitor.
FIG. 1 is a schematic of a differential capacitor 10 that has stationary outer electrodes 12, 14 and a movable inner electrode 16. Electrodes 12, 14 are coupled to DC voltage supplies that provide 3.4 volts and 0.2 volts, respectively, and electrode 16 has a DC voltage of 1.8 volts. Electrodes 12, 14 are also coupled through respective capacitors C1, C2 to clock signals 18, 20. Signals 18, 20 have a small peak-to-peak voltage of about 0.6 volts and have the same frequency, but each is 180.degree. of out of phase from the other. As electrode 16 moves toward one or the other of electrodes 12, 14 (e.g., in response to an external force), a signal appears on electrode 16 and at its output node 22. This signal follows one or the other of signals 18, 20 and has a peak-to-peak amplitude that is proportional to the displacement of electrode 16.